How could I return a nullable value in typescript

Question

in an NPM module I use typescript

  "devDependencies": {
    "@types/node": "^8.0.0",
    "typescript": "^2.8.1"
  }

and I want to return a private nullable parameter using a public method. Please refer to the axample below. The error I see is

Property 'string1' has no initializer and is not definitely assigned in the constructor.

If I assign an undefinded in the constructor I got the error

[ts]
Type 'string | undefined' is not assignable to type 'string'.
  Type 'undefined' is not assignable to type 'string'

How should I do this in typescript, I´m from the c# side :)

export class HowToDoThis {

    private string1?: string;

    public constructor() {

        //this.string1 = undefined;
    }

    public add2String1(content: string) {

        this.string1 += content;
    }

    public getString1(): string {

        return this.string1;
    }
}

Answer 1

You can define

private string1: string | undefined;

Answer 2

The error message you are getting Type 'string | undefined' is not assignable to type 'string' isn't because you assigned this.string = undefined in the constructor, it is because you defined getString1() as returning string and you didn't check that this.string1 was in fact a string.

You can either change getString1() so it does in fact always return a string, or you could make it return string|undefined, or much simpler you could just initialise string1 to an empty string and never have it undefined.

So this works:

export class HowToDoThis {
    private string1?: string;

    public constructor() {
        this.string1 = undefined;
    }

    public add2String1(content: string) {
        this.string1 += content;
    }

    public getString1(): string {
        return this.string1 || "";
    }
}

But this would be better if only because calling add2String1('foo') won't give you the string 'undefinedfoo':

export class HowToDoThis {
    private string1: string = "";

    public add2String1(content: string) {
        this.string1 += content;
    }

    public getString1(): string {
        return this.string1;
    }
}

And this is best of all (don't use getter functions in typescript, you can always create a get property later if you need to do something when a property is accessed):

export class HowToDoThis {
    public string1: string = "";

    public add2String1(content: string) {
        this.string1 += content;
    }
}